A physicist is a scientist who has specialized knowledge in the field of physics, the exploration of the interactions of matter and energy across the physical universe. Physicists study a wide range of phenomena in many branches of their field, spanning all length scales: from sub-atomic particles of which all ordinary matter is made (particle physics), to physical systems, to molecular length scales of chemical and biological interest, to cosmological length scales encompassing the Universe as a whole. Physicists generally are interested in the root or ultimate causes of phenomena, and usually frame their understanding in mathematical terms. Physicists can also apply their knowledge towards solving real world problems and/or developing new technologies (also known as Applied Physics or En